1. Field of the Invention
The present invention relates to a multi-hood system for correcting individual distortions in a reduction projection lens for use in semiconductor exposure, according to rates of the distortions in individual regions of the lens.
2. Discussion of the Related Art
As shown in FIG. 1, which illustrates a conventional exposure system used for photomasking, a fabrication process of a semiconductor device includes, in general, a reticle stage 8 for mounting a reticle 7, a field lens 9 for adjusting the magnification of a lens, a reduction projection lens 1 for determining a size of an exposed pattern, and a wafer chuck 10 for mounting a wafer 5, arranged in order from top to bottom. A temperature controller 6 is provided with an air supply hood 2a around the outer circumference of the reduction projection lens 1 of the exposure system, and controls a temperature of the air supplied to the air supply hood 2a through a supply line 4a in order to maintain the temperature in the air supply hood 2a constant.
Accordingly, in the conventional exposure system, light from a light source reduces a pattern area of the reticle 7 at a predetermined ratio and projects a pattern onto the surface of the wafer 5. In this instance, the pattern formed on the surface of the wafer 5 is distorted due to lens aberrations, such as astigmatism existing in the reduction projection lens 1. In order to cope with this inaccurate formation of the pattern on the surface of the wafer 5 by the light passed through the field lens 9 and the reduction projection lens 1 along the light path shown in FIG. 1 due to the lens aberration, the temperature of the reduction projection lens 1 is elevated or lowered by controlling a temperature of the air supplied to the air supply hood 2a provided around the reduction projection lens 1. The controlled temperatures expand or contract the reduction projection lens 1 to minimize the distortion.
However, when the distortion of the pattern formed on the surface of the wafer 5 is corrected by controlling the temperature of air supplied to the air supply hood 2a to expand or contract the reduction projection lens 1, the uniform distribution of air supplied around the reduction projection lens 1 along the circumference of the air supply hood 2a causes the reduction projection lens 1 to expand or contract evenly in its entirety. Accordingly, if the rate of distortion of the reduction projection lens 1 is different from region to region of the lens' outer circumference, then a perfect correction of the distortion of the pattern formed on the wafer is not possible. For example, as shown in FIG. 3a, if rates of distortions in all regions of the pattern formed on the wafer are identical, then air of the same temperature is supplied throughout all the circumference of the lens thus eliminating the distortions on the pattern. That is, if the actual pattern shown with a solid border was formed greater than the desired size shown by dotted borders in FIG. 3a, then air of a lower temperature is supplied into the air supply hood 2a for restoring the size of the pattern to normal as shown in FIG. 3b. However, different from the aforementioned case, if there is a region "L" on the pattern which has a distortion rate different from another region such as in FIG. 3c, then air of the same temperature supplied to all regions of the reduction projection lens 1 cools down the reduction projection lens 1 uniformly, as shown in FIG. 3d. The region which has a uniform distortion rate is corrected of its distortion, but the region which has a different distortion rate still has a distortion. Each of the dotted lines in FIGS. 3a and 3c is provided to show a normal sized pattern for comparison to a wrongly formed pattern. Although only the case in which the pattern size is expanded greater than normal has been explained herein in connection with FIGS. 3a.about.3d, a situation in which the pattern size is contracted smaller than normal will have incorrectable distortions when the rates of distortions are different depending on the lens regions. In conclusion, due to the limit in the deviation correcting capability of the conventional distortion correcting device, there have been many problems with inaccurate formation of the pattern by exposure during photo-masking, resulting in a low exposure process yield.